Masonry Design In-Plane-Bending LRFD failure but ASD Passing 1

[EngStuff]

I have been looking around and couldn't find a specific example or explanation of why this is happening, My situation is I am analyzing an existing 12" non reinforced masonry wall in both strength and stress method for shear.

The wall is a 12" unreinforced masonry structure that has a height of 17 feet, and a length of 27 feet. f'm = 1,500. it is nonbearing wall with a shear load from wind of about V(ultimate)= 13554k or 0.502k/ft.

I did it on RISA 3D and noticed in LRFD it was failing, and in ASD it was good. I looked through the code check and determined that it was the in-plane bending due to shear that was causing the failure in strength design. I ended up reanalyzing only by hand in ASD and it works good.

Am I doing something wrong, if not, what am i not understanding correctly?

here are 2 images from RISA, both.

strength design

stress design

 

[XR250]

There is no bending stress in the "stress design". How come?

 

[EngStuff]

There is no bending stress in the "stress design". How come?

I think there is, it's the fb =.032 ksi. When i did my hand calculation, for ft= Mc/I, I'm getting about 40.17 lb/in2 which is about .040ksi Unless I am incorrectly assuming fb=ft=Mc/I.

 

[XR250]

You have a unity check of 1.306 for strength design and 0 for stress design.
It shows 32 ksi capacity for in-plane bending - that is not even close to being correct. Check ACI530 for allowable flexural tension stress.

 

[Celt83]

32 ksi is correct for the Allowable steel stress.

For Strength design is it performing a strain compatibility analysis, but it does not appear to be accounting for the axial compression at location 0?

For Stress design k*d = 0 which implies there is no net tension on the bottom of the wall.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[Celt83]

Oh I see now you indicate that this is an unreinforced wall, I would focus in on the strain compatibility approach for unreinforced masonry in ACI 530.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[EngStuff]

It shows 32 ksi capacity for in-plane bending - that is not even close to being correct. Check ACI530 for allowable flexural tension stress.

I am not a risa 3d expert, so don't hold me to what I am about to say, but i think that even though risa 3d offers unreinforced wall check, the program still analysis using steel at the boundry layers. this is why I get a Fs = 32 ksi, I am ignoring that and only using fb = 0.032 ksi (32psi) and comparing that to the ACI530 vaule of 0.050ksi (50psi) This is why i decided to do it by hand after.

Oh I see now you indicate that this is an unreinforced wall, I would focus in on the strain compatibility approach for unreinforced masonry in ACI 530.

I have to ask this, but If doing it by stress checks out, why bother going through with strain capacity if it's failing? My issue is this is an existing wall.

 

[Celt83]

It may be a limitation of RISA's calculation model for either stress or strength methods that this particular model doesn't fit within their assumptions making RISA's calc invalid. You'd probably need to reach out to the tech support to get a good response based on their application of the code provisions.

It's not entirely clear from the calculation output but the strength provisions also include moment magnification which may be leading to the large disparity in utilization. although doesn't appear to be a factor since you said this is wind load and Mu = Ms/0.6.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[XR250]

This is a perfect example of why hand checks should be utilized.

 

[PMR06]

Those outputs looks like they are from envelope runs. Are you sure both the LRFD and ASD envelope runs have all the appropriate load combinations clicked to run? And do the load combinations contain all the correct basic load cases? For example, does LC 17 include the axial load?

 

[EngStuff]

Those outputs looks like they are from envelope runs. Are you sure both the LRFD and ASD envelope runs have all the appropriate load combinations clicked to run? And do the load combinations contain all the correct basic load cases? For example, does LC 17 include the axial load?

correct it is ran as an envelope. LC 17 does include the Axial load. it's .9*DL + 1*WL For strength. I had went back and forth with turning on and off the correct Load combinations for whichever way I changed the code check requirements. For ASD I unchecked the strength, for LRFD I unchecked the Service loads.